The present invention relates to an antenna system and in particular to a telemetry antenna system using a lightweight Luneberg lens as the aperture.
It is especially important in todays technology to be able to receive a signal emitting from a spatial object such as a target within a particular predetermined spatial volume. For instance, re-entry ballistic missiles transmit telemetry data, tracking data and impact location data for strategic missile testing. Such data can be collected from a variety of airborne objects or vehicles. Further, it is important to gather navigation data from satellite-to-ground stations such as mobile vehicles or ships and the like. In addition, fixed position satellites require ground stations that must receive data from more than one satellite simultaneously. Also, in border surveillance it is necessary to cover a wide area and detect anything that moves within a volume that includes that particular area.
In a system for receiving telemetry data during the terminal phase of a re-entry vehicle (RV) for missile targeting, it is important to have a spatial volume covered or scanned that is wide in azimuth but narrow in breadth or elevation so that a system that is airborne may focus on a predetermined volume along the horizon and detect and locate any RVs re-entering that particular volume. Further it is important in an airborne system that it be a nonphysical scanning telemetry antenna system because the system has to be operated from an aircraft where physical movement of the antenna would be extremely limited. A desired sector volume coverage would include an area of 11.degree. to 12.degree. in elevation and 45.degree. in azimuth. Further, in such system it is important that the angular or azimuth coverage be achieved in at least two modes of operation. The first mode is a combined feed mode for which a wide, single fan beam will be formed by the coherent summing of all of the feeds and secondly a switched feed mode for which narrow, individual, but overlapping, beams will be formed.
In addition, the antenna system must receive and process the entire operating band, in this case 2200 to 2300 MHz, to determine the antenna beam containing the strongest signal without knowledge of the frequency channel being used by the transmitting carrier within the operating band. Since there are a large number of channels within the telemetry band, the antenna system, in the automatic mode, must perform its automatic beam switching function without a prior knowledge of the telemetry frequency channel to be used for a particular mission or the assignment of the frequency channel for a particular airborne object such as a re-entry vehicle.
Electronically scanned arrays could meet such requirements but are extremely expensive and have many undesirable features. For instance, in the U.S. Pat. No. 3,487,413 to M. W. Shores, antenna elements are grouped in parallel column fashion and firmly attached to a Luneberg lens over an arc which is approximately equal to 1/2 the desired "look" angle. Elements of the array are energizable in accordance with a programmed sequence. Thus it provides a sequential, not simultaneous, lobe coverage of wide angles by means of programmed switching of element feeds one at a time. Further, the antenna may be mechanically rotated to a desired position by way of an axle extending axially through the Luneberg sphere in order to provide a full hemispherical look angle.
A Butler matrix coupled to a planar array of coherently summed elements in the vertical rows could possibly also meet the requirements but again the cost and problems including the difficulty of achieving better side lobes than that produced by uniform illumination are not acceptable. A fixed paraboloid reflector aperture with multiple feeds to achieve azimuth coverage and a method of switching the feed to obtain the maximum gain of the aperture when required is also possible. However, with four or more S-band orthogonal mode feeds in front of a 30 inch diameter paraboloid reflector and even with the size reduction of the feeds brought about by dielectric loading, the aperture blockage remains prohibitive. A feed system offset from half of a paraboloid reflector could be used but the problem associated with illuminating the reflector from other than its focal point is significant as is the feed and reflector development that is required.
A lightweight Luneberg lens was discovered to be the ideal aperture configuration especially for airborne telemetry antenna system applications. The Luneberg lens is a spherical dielectric lens made with stepped dielectric constant materials which vary radially in dielectric constant from 2 at the center to 1 at the surface. For the true Luneberg lens, the focal point is on the back surface of the lens which is away from the signal source. A common version of the Luneberg lens (referred to some times as a Morgan lens) is one whose design has been modified slightly to make the focal points fall on a spherical surface just off the lens surface to facilitate coupling to the phase center of the feed system. A significant advantage of the use of the spherical dielectric lens, particularly for small apertures, is that feed blockage of the apertures is eliminated. Also, all feeds can be placed of focal points of the spherical lens aperture. All feed beams are also on the boresight of the aperture and, therefore, do not have the gain reduction present for off-boresight beams of phased array antennas. In addition, simple feeds such as open-ended wave guides produce ideal low sidelobe antenna patterns.
By testing different available feeds, it was concluded that a 44.degree. azimuth coverage and 11.degree. elevation coverage could be obtained from a 30-inch Luneberg lens by simultaneously and coherently summing the outputs of four feeds. This gives a gain reduction of about six dB relative to a single feed in order to obtain the fourfold increase in azimuth beam width.
It was found necessary, if the gain was to be kept above 20 dB, that beam switching must be employed using a unique technique that determines in which feed the beam object or target is located.
Thus the novel antenna system design uses a 30-inch diameter lightweight Luneberg lens equipped with four feeds in the azimuth plane at the equator to achieve single beam patterns or selective multiple beam patterns. The feeds are quad-ridged circular devices with orthogonal linear polarization outputs which are converted to simultaneous left and right-hand circular polarization using 90.degree. hybrid couplers. The operator may manually select any one of four single beams covering 11.degree. azimuth by 11.degree. elevation, two beams combined for 22.degree. azimuth by 11.degree. elevation sector coverage, or four beams combined for 44.degree. azimuth by 11.degree. elevation sector coverage. An automatic mode permits the full gain of a single beam (about 22 dB) to be attained and switches automatically to the RF feed containing the greatest signal in the 44.degree. by 11.degree. sector. Information for the automatic switching is achieved by comparing the signal power output from radiometer receivers coupled to each feed. If desired, one may be used for each orthogonal polarization output for each of the four antenna feeds. The automatic RF switching is achieved by PIN-diode switches in ten nanoseconds.
Further, the output of the feed ports are coupled to gain and phase matched Gallium Arsenide (GaAs) FET low noise preamplifiers (LNA's) through external limiters. The limiters protect the LNA's against accidental RF input of up to six watts average power. The amplification prior to the 90.degree. hybrid couplers, used to convert the two orthogonal polarizations to left and right-hand circular polarizations, substantially reduces the noise figure contribution that would otherwise be associated with the insertion loss of the hybrids. Directional couplers are used to divert a tenth of the power from both horizontal and vertical polarization outputs from each feed channel to the radiometer receivers. These receivers have a 120 MHz predetection RF bandwidth to assure that the signal amplitude is sufficient at the band edges of the 2200 to 2300 MHz telemetry band. The output cf the radiometers is fed to comparators and logic circuitry which select the feed with the most signal power and produce outputs which drive a 1.times.6 PIN-diode switch to select the feed which produces the greatest signal. The PIN-diodes switch the RF signal in ten nanoseconds. This switching time is small compared to the width of the PCM pulses received. Therefore, decommutation equipment will not miss a single bit during the automatic hand over from one feed beam to another. The use of the PIN-diodes and two-way and four-way combiners achieve four modes of operation. These modes include selection of any one beam, selection of two coherently summed beams, four coherently summed beams, and automatic switching from one beam to another.
Thus, the present invention uses a Luneberg lens aperture for producing multiple beam patterns covering a predetermined spatial volume and in which coherent combining of the antenna feed outputs is utilized to vary beam width and in which automatic scanning of the multiple beams may be accomplished to lock on to the beam containing the signal from a spatial object.
The novel invention utilizes control means coupled to the outputs of each of the antenna feeds for determining which of the feeds is producing the greatest power output and generating a corresponding control signal which is utilized by a switching network to enable coupling to the receiver only the output from the feed producing the greatest power.